When painting Q550NH weathering steel, surface pretreatment must carefully balance corrosion protection and paint adhesion while preserving the steel's inherent weathering properties (if desired). Here are the critical requirements:
1. Sandblasting (Abrasive Cleaning)
Standard: Achieve SA 2.5 (ISO 8501-1) or SSPC-SP 10 (Near-White Metal) cleanliness.
Abrasive Choice:
Use non-metallic abrasives (e.g., garnet, alumina) to avoid iron contamination.
Avoid copper slag or steel grit (may embed reactive particles).
Profile Roughness: 50–75 μm (anchor pattern for paint adhesion).
2. Mill Scale & Rust Removal
Complete Removal: Essential even if Q550NH's patina is desired beneath paint.
Chemical Cleaning:
Phosphoric acid (5–10% solution) to convert residual rust to iron phosphate.
Rinse thoroughly to prevent paint failure.
3. Preserving Weathering Properties (Optional)
For Hybrid Systems: If partial patina is retained (e.g., aesthetic edges):
Spot Sandblasting: Clean only areas to be painted.
Rust Stabilizers: Apply tannic acid or proprietary converters to untreated zones.
4. Priming & Paint Compatibility
Zinc-Rich Primers: Use low-zinc or zinc silicate primers to avoid galvanic interference with Cu/Cr.
Epoxy/Polyurethane: Must be breathable to allow moisture escape (avoid blistering).
Testing: Verify adhesion via cross-cut test (ISO 2409) and salt spray resistance (ISO 9227).
5. Environmental Controls
Humidity: Paint within 4–6 hours after blasting (max 85% RH).
Temperature: Apply between 10–35°C to ensure curing.
